Conventionally, there has been well known an automatic transmission that forms a desired shift stage by a combination of engagement states of frictional engagement elements such as a clutch and a brake. In this automatic transmission, a shift is performed in accordance with a degree of pressing down the accelerator (a foot pressing amount of an accelerator pedal) or a vehicle speed. For example, in a case where a degree of pressing down the accelerator increases abruptly during running of a vehicle, a down shift (referred to as a power-on down shift in the following description) or the like is performed.
When an accelerator is subjected to an OFF operation after commencement of the power-on down shift, a torque to be inputted from an engine decreases. Consequently, a difference between a turbine rotation speed and a synchronized rotation speed after the shift does not narrow, resulting in a plateau of the shift. In order to increase the turbine rotation speed of the automatic transmission to the synchronized rotation speed, sweep control is executed for increasing an engaging pressure for a frictional engagement element on an engagement side at a predetermined rate of change. The sweep control allows gentle engagement of the frictional engagement element on the engagement side, leading to suppression of a shock and progress of the shift.
As such shift control, for example, Japanese Patent Laying-Open No. 2004-125075 discloses a shift control method of an automatic transmission, which realizes a shockless shift by resolving following delay of a hydraulic pressure for an engagement element on an engagement side in a case where a power-off down shift is performed after commencement of a power-on down shift. In this shift control method, when a command of the power-off down shift is issued, an initial pressure is supplied to an engagement element. Further, at a time point when an input rotation speed becomes higher than an input rotation speed at a high shift stage by a predetermined value, a hydraulic pressure for the engagement element is feedback controlled such that the input rotation speed increases at a target rate of change. This control method includes a step of detecting a fact that a command of a power-off down shift is issued after commencement of a power-on down shift, a step of maintaining a certain pressure in the vicinity of an initial pressure for an engagement element irrespective of a change of an input rotation speed only during a predetermined period of time from the time point at which the command of the power-off down shift is detected, and a step of feedback controlling a hydraulic pressure for the engagement element such that the input rotation speed increases at a target rate of change after a lapse of the predetermined period of time.
According to the shift control method disclosed in the publication described above, it is possible to resolve following delay of a hydraulic pressure for an engagement element on an engagement side and to realize a shockless shift.
However, if a driver performs the ON operation on the accelerator again while the sweep control is executed, the torque inputted from the engine to the automatic transmission increases. Consequently, there is a problem that the turbine rotation speed of the automatic transmission increases more than necessary unless the engaging pressure for the frictional engagement element on the engagement side increases sufficiently. If the turbine rotation speed increases more than necessary, there is a possibility that a shock is generated upon formation of the shift stage after the down shift. The publication described above has no consideration to such a problem.